Late Ordovician-early Silurian Sedimentary Change And Event Sedimentation Study

The Ordovician-Silurian mass extinction event was the first extinction event in Phanerozoic.Among the five mass extinctions in Phanerozoic,this extinction event was the second largest extinction event in the five major extinctions,resulting in the extinction of many marine biological species such as brachiopods,graptolites,trilobites,etc.It mainly includes two acts,namely,the first act occurred at the end of Cady?D.mirus graptolite belt?,which is consistent with the starting time of the Hanant ice age,the early stage of Hanant?N.extraordinarius graptolite belt?,and the second act occurred at the beginning of the late stage of Hanant?from N.extraordinarius graptolite to N.persculptus graptolite belt?,where the Gondwana ice cap began to melt.However,this extinction event is different from other Phanerozoic periods,mainly in which the triggering mechanism of the Ordovician-Silurian mass extinction event is still unclear.However,for the discussion of the causes of extinction,the current hypothesis mainly includes crater events such as Hernanite glacial events,sea level rise and fall,climate change,redox changes,gamma ray bursts,fire meteor impacts and large igneous provinces.Based on the study of C and O isotopes and major,trace and rare earth elements such as Al,U,Pb,Ce and Ba in the strata of Wufeng Formation of Late Ordovician to Longmaxi Formation of Early Silurian in Laolin section of Dejiang area in northern Guizhou,the correlation between volcanic activity of Late Ordovician and the sedimentation and sedimentary changes of Late Ordovician to Early Silurian events is discussed.Near the Late Ordovician-Early Silurian world line,the measured U-Pb weighted age of zircon is 444.7±4.5 Ma,which exactly corresponds to the maximum positive value of the positive deviation of carbon isotope in the first act?from the end of Cady to the early part of Henant?.This indicates that the material source of the age element deposited in the event is mainly from the ²³⁸U and ²⁰⁶Pb isotope deposited from volcanic eruptions into the atmosphere,rather than from terrigenous U and Pb elements.Combined with the main,trace and rare earth elements,it has the characteristics of indicating significance to the changes of sedimentary environment.The changes of C,Be,Sr,Zn,Ba and other elements are compared with various indicators of sedimentary environment to judge the response degree of terrigenous debris input,redox environment,chemical weathering and other effects during the event deposition process in northern Guizhou and reveal its formation mechanism.The results showed that the Yangtze block in the Late Ordovician-Early Silurian period was in a deep-water environment of occlusion,detention and anoxia,and there were two obvious ¹³C shifts near the boundary in this period.The carbon isotope in the first act has a large positive deviation from the end of Cady to the early stage of Henant,which corresponds to the peak of Henant ice age and coincides with the time of D.mirus graptolite belt in the late stage of Late Cady in the Ordovician-Silurian mass extinction event.The second negative migration occurred in the upper part of N.extraordinarius-N.ojsuensis graptolite belt in the middle of Henant,which was basically consistent with the second act of the extinction event?late Henant?.This also indicates that the biological extinction event may be related to the intense volcanism at that time.In this thesis,the formation of tuff is finally attributed to volcanic origin.This study updated the boundary of the Late Ordovician Wufeng Formation-Early Silurian Longmaxi Formation stratigraphic system in northern Guizhou,and also provided reliable reference for the division and correlation of the Ordovician-Silurian stratigraphic boundary for other regions of the world.